22
TD_MRN3-3_08.06_GB
5.4 Protection
parameter
5.4.1 Setting parameters for under
voltage
characteristics
For the under voltage detection, the
MRN3-3
has two
under voltage characteristics (limit curves) that can be
independently set and that each provide 5 characteris-
tic definition points. All individual setting points are de-
fined by two parameters: one voltage value (1) (in
Volt) and a time-value (2) (in seconds).
Both characteristics can be optionally defined to either
provide a warning or a tripping function. The differ-
ence lies in the varying display indication. In case of
warning, the display retains unchanged, for tripping,
the abbreviation « TRIP » will be indicated.
For characteristic setting point 1, only the value 1, i.e.
the voltage will be adjusted since excitation of the step
always starts as soon as the failure occurs. A fault in-
cident is detected when the voltage is below the
threshold value U<start and it will be stopped as soon
as the voltage range U<5 had been exceeded.
At the moment when the voltage falls below the
threshold value U<start, the
MRN3-3
will initiate the
tripping timer. The present voltage is compared with
the adjusted characteristic after expiry of each one
measuring cycle*. If the voltage at time x is below the
appropriate characteristic value, the
MRN3-3
relay
will trip. When the U<Start parameter is set to EXIT,
the characteristic is out of service and all relevant set-
ting values will be faded out.
* One measuring cycle is 20 ms at 50 Hz and it takes 16.66 ms
at 60 Hz.
0
20
40
60
80
100
120
-2
-1
0
1
2
3
4
5
Char 1
Aux. line
Char 2
U-voltage
range
failure
incident
U/Un interlinked voltage
time in s
U<4
lower value of
the voltage range
U<1
U<2
U<3
U<5
U<1
U<2
U<3
U<4
U<5
Char. curve 2
Char.
curve 1
U<Start
Figure 5.4: Trend of two-characteristics